(1) Field of the Invention
This invention relates to a process for producing a thermoplastic polymer composition comprising a specific olefin polymer.
(2) Description of Related Art
As a cross-linking rubber component for a thermoplastic olefin elastomer, there has heretofore been generally used an ethylene-propylene-diene rubber (EPDM). The diene component in the polymer chain of EPDM is necessary for enhancing the cross-linkability; however, the environmental deterioration resistance becomes insufficient owing to the presence of the above diene component, and an improvement in quality has been desired.
On the other hand, even in the case of an ethylene-propylene rubber (EPR) which does not contain the diene component, it is possible to cross-link the same with a radical initiator such as an organic peroxide or the like; a polyfunctional unsaturated compound; or the like. However, since EPDM and EPR which have been used in the conventional thermoplastic olefin elastomers as mentioned above are produced using a so-called "multisite catalyst" of the Ziegler type, many polymerizing active sites are present therein, the molecular weight distribution is broad, the ultra low molecular weight component is contained in a considerable amount, and hence, the balance between their own flexibility and mechanical strength is inferior.
Moreover, as to the Ziegler type EPR and EPDM, the polymer structure which can form pellets is limited and EPRs or EPDMs having the other structures are in the veil form and are complicated in handling, and hence, are not desirable. In addition, even EPR and EPDM which form pellets have a problem of blocking due to the ultra low molecular weight component, and hence, they are not desirable, either.
The present inventors have already presented, in JP-A-9-104,787, a thermoplastic composition prepared by cross-linking, with a radical initiator, a composition containing an olefin polymer produced using a metallocene type catalyst; however, it has been one which still leaves room for improvement in physical properties.
Furthermore, WO96/35,751 discloses a process for producing a vulcanized, thermoplastic, elastic composition by kneading, in the presence of a cross-linking agent, a composition containing an elastic ethylene copolymer produced using a metallocene type catalyst. However, the composition obtained still leaves room for improvement in physical properties.